1. Field of the Invention
The present invention relates to an optical information recording method for recording multilevel information on an optical information recording medium such as an optical disk, an optical information recording apparatus such as an optical disk apparatus that implements this optical information recording method, a laser control circuit that controls the irradiation of a laser beam, a wobble signal detection method for detecting a wobble signal in multilevel recording, and a servo signal detection method for detecting a servo signal in multilevel recording.
2. Description of the Related Art
These days, CD drives and DVD drives are gaining recognition as optical information recording apparatuses for recording data on an optical information recording medium such as a CD or a DVD. Also, the data size of files being handled is becoming larger and, consequently, greater capabilities are being demanded in the optical information recording apparatus to achieve a larger capacity in the optical information recording medium.
One way of increasing the capacity of the optical information recording medium is to use a multilevel recording technology (e.g., Japanese Patent Laid-Open Publication No. 2001-84592). According to the multilevel recording technology, cells having a predetermined length in the circumferential direction of an optical disk track are defined as imaginary recording units (cells), and a laser beam is irradiated on the cells so that a mark is recorded on each of the cells. Herein, the circumferential direction length of the cells is generally no more than the optical resolution of the laser beam spot. Information is embedded in an area of the mark recorded on each cell. By distinguishing multiple levels of reflection signals from the cells according to the respective mark areas, multilevel information can be extracted from the cells.
In the conventional CD drive or DVD drive, which uses a bi-level information recording method, information is embedded in a recording mark having a length that is greater than the optical resolution. Thus, according to this method, a limit to reducing the beam spot diameter prevents the realization of a larger capacity in the optical disk. Specifically, the beam spot diameter is physically determined by the wavelength of the laser beam and the numerical aperture (NA) of the lens. Therefore, an attempt at reducing the beam spot diameter has to rely on increasing the wavelength of the laser short wave and increasing the NA of the lens. On the other hand, according to the multilevel information recording method, multilevel information is recorded on cells having lengths that are no more than the optical resolution, and therefore, the capacity of the optical disk can be reduced without relying on reducing the beam spot.
A representative laser emission waveform for multilevel information recording using phase change media includes a write pulse with high light intensity for melting the recording layer of the optical information recording medium, an off pulse with low light intensity for rapidly cooling the recording layer to form a mark, and a space pulse for forming a space on the recording layer (or for erasing an existing mark in the case where re-writing is possible).
In this example, the off power period takes up a large portion of the mark formation time, and light emission is very weak during this period. Namely, the light intensity of the off power period is arranged to be no greater than a playback power. Particularly, the light intensity of the off power period immediately after the write power period is preferably set as low as possible in order to enhance the cooling effect (it is even possible to have no light emission at all). However, when the laser beam output is low, noise is increased due to instability in the light emission state created by influences from the returning (reflected) light. Additionally, the reflection signal intensity is also weakened, resulting in vulnerability to influences from factors such as circuit noise.
Normally, during playback, the playback power has to be set relatively low in order to protect the marks from degradation. Thus, a high frequency modulator (HFM) that can reduce the noise of the laser beam may be used, or the amplification rate of a first stage detection circuit may be increased so that sufficient signal amplitude can be secured and the signal components are not confused with circuit noise, for example.
On the other hand, during recording, the amplification rate of the first stage detection circuit has to be decreased in order to prevent saturation of the signals during the write power period or the space power period in which the light intensities are high so that the signals can be accurately detected. Further, the high frequency modulator is normally turned off during recording and, thus, the signal components obtained during the off power period (weak light emission period) are likely to be confused with noise of the laser beam and circuit noise so that most signal components cannot be detected during this period. Additionally, since the write pulse period is very short, stable signal detection cannot be performed during this period.
However, even during recording, there are signals such as a servo signal and a wobble signal that need to be detected. The servo signal indicates position information for guiding the beam spot to follow a desired track, and the wobble signal indicates information embedded in the recording medium such as address information and rotation information.
In recording multilevel information using a conventional optical information recording apparatus, signal detection is possible when the laser emission waveform corresponds to the space power period in which a sufficient amount of light can be secured at a stable rate; however, during the write power period and the off power period accurate signal detection is impossible for the reasons explained above. Further, since a mark and a space are generated at the same probability, an effective time period for the signal detection is about half the recording time. Consequently, this may lead to degradation in the detected signal quality.
Also, it is noted that in multilevel recording, the recording waveform and its timing are different from those used in the bi-level recording and, therefore, the conventional laser control circuit (LSI: large-scale integration) cannot be used. In bi-level recording, information is represented by the length of the mark, and thereby, the recording emission interval changes depending on the information sequence. When a minimum mark/space length is designated as 3T (T representing the base period) such as in a CD or DVD, a space of length 3T that requires no waveform processing is inserted between marks requiring complicated waveform processing. The laser control circuit handles this space period as preparation time for the emission waveform processing of the next mark.
On the other hand, in multilevel recording, marks of various sizes (areas) are each recorded at the center of a cell, and thereby, the recording light emission has a regular period. However, a space is not provided between the cells and waveform processes have to be successively performed.
In summary, the two major differences between the laser control circuit for bi-level recording and the laser control circuit for multilevel recording are:    1) the positioning of the recorded marks (irregular/regular)    2) the arrangements for waveform processing of the recorded marks (insertion/non-insertion of space between the recorded marks as preparation time)
Thus, the conventional laser control circuit for bi-level recording cannot be used for the multilevel recording, and measures have to be taken to adapt the laser control circuit for multilevel recording.
Further, there is a problem in that a wobble signal and a servo signal suitable for multilevel recording cannot be properly detected in the conventional art.